There are many situations, where it is desirable to provide a method of delaying or preventing flow separation between a flowing medium and a flow control surface in regions where the boundary layer of the flow medium due to the profile of the flow control surface is subjected to pressure gradients, which are sufficient to cause flow separation.
When a viscous fluid passes over a wind turbine blade towards the trailing edge, the fluid flows from a region with low static pressure to a region with high static pressure, in the process being subjected to an adverse pressure gradient. This in turn results in forces, which tend to retard the boundary layer, which can be strong enough to arrest or reverse the flow, which can cause the fluid to separate and behave in a non-predictable manner. This causes an increase in drag due to the cross-sectional area of separated flow in the wake of the flow control medium, which in turn reduces the lift of the wind turbine blade and even may cause the blade to stall.
It is well-known to delay or prevent flow separation by mixing free flow with the boundary layer by use of vortex generators protruding from the flow control surface, i.e. from the surface of the wind turbine blade. There is a large number of different vortex generator types, such as of the vane type, see for instance WO 01/16482, or vortex generators formed as delta shaped protrusions as shown in WO 00/15961. However, all of these vortex generators are encumbered with a drawback of relatively high drag. Furthermore, these vortex generators, which are usually mounted on the surface of the wind turbine blade after production of the blade, have a tendency to break off during transport, which may seriously impair the functionality of the blade.
U.S. Pat. No. 4,455,045 describes an alternative means to maintain a flow of a flowing medium attached to the exterior of a flow control member, where an essentially triangular shaped channel is submerged in the surface of the flow control member. The triangular shaped channel has an apex portion facing the flow of the flowing medium, and the channel emerges at the surface of this apex portion.